Influence of Mehlich-3 extractable aluminum on phosphorus retention in organic soils

Phosphorus (P) regulations in North Carolina (NC) assume that organic soils have little ability to retain P, and losses of P from these soils are of concern for the health of aquatic ecosystems. However, recent research has indicated decreased water-soluble P (WSP) in organic soils in the presence of high Al concentrations. Our objectives were to determine (i) the concentrations of organic matter (OM), and Mehlich-3 Al and Fe in deep and shallow organic soils in NC, and (ii) how these factors affect P retention. We sampled four organic soil series and determined Mehlich-3 P, Al, and Fe (M3P, M3Al, M3Fe), WSP, total P, pH, particle size distribution, and OM. Water-soluble P and M3P were also measured in a 21-day incubation study, in which P was added at a rate equivalent to 150 kg P ha(-1). The main cation responsible for P retention was Al, and the mean topsoil M3Al concentrations (1926 mg kg(-1)) in these organic soils were close to three times higher than those observed in another study of mostly mineral NC soils. Mehlich-3 Fe was not significantly related to WSP. Retention of added P was negatively related to OM and positively related to M3Al. The ratio of OM and M3Al was the best predictor of WSP and retention of added P. The results from this study indicate that organic soils can retain P more strongly than previously thought, and their ability to retain P is best described by the ratio of OM and M3Al.